1. Field of the Invention
The present invention generally relates to a surface acoustic wave (hereunder sometimes abbreviated as SAW) device for achieving frequency stabilization and frequency control by using surface acoustic wave, and more particularly to a SAW device whose frequency-temperature characteristics are noticeably improved.
2. Description of the Related Art
Among conventional SAW devices, one-port SAW resonators and two-port SAW resonators are well known as those used for frequency stabilization and frequency control. Especially, a SAW resonator using an ST-cut quartz crystal as a substrate has a zero temperature coefficient at ordinary temperature and is of high accuracy. However, the secondary temperature coefficient .beta. of the aforementioned ST-cut quartz crystal substrate is -3.3.times.10.sup.-8 /.degree. C..sup.2, so that the frequency-temperature characteristics or change is approximately .+-.60 ppm in the temperature range of -35 to 85.degree. C., at which mobile communication systems or devices are used. Therefore, the SAW resonator using an ST-cut quartz crystal substrate has inadequate characteristics and thus cannot be used in a mobile communication device. There is a known method for improving the frequency-temperature characteristics of a SAW device against this by which two SAW devices are used by being electrically connected in parallel with each other (see Japanese Unexamined Patent Publication No. 55-121729 and Japanese Unexamined Patent Publication No.58-39105).
However, in the case of the aforementioned conventional method, the coupling coefficients of each of the two SAW devices is determined by the capacitance of a capacitor (the load capacitance CL), which is an external or outboard device, so that the frequency-temperature characteristics are realized. Thus, in the case of an oscillation circuit comprising two SAW devices, the frequency of a signal generated by the circuit is determined by the same capacitance CL. Consequently, the circuit lacks flexibility in regulating the frequency. Further, in the case of the SAW devices used as a filter, the filter lacks flexibility in setting terminal impedance and is, therefore, very inconvenient.
It is, accordingly, an object of the present invention to improve the frequency-temperature characteristics of a SAW element or device and to provide a SAW device, which excels at frequency accuracy and is very convenient, to the market.